LOSS AND LOSS ADJUSTMENT RESERVES - (Schedule of Incurred Losses and ALAE, Net of Reinsurance) (Details) $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)
Homeowners [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 1,084,095
Homeowners [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	30,651	$ 30,602	$ 30,459	$ 29,945	$ 29,407	$ 28,968		$ 27,895	$ 26,151	$ 25,056	$ 24,825
IBNR & development on reported claims	$ 66
Cumulative number of reported claims	2,393,000
Homeowners [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 26,394	26,394	26,143	25,957	24,582	23,932		23,007	21,344	20,492
IBNR & development on reported claims	$ 33
Cumulative number of reported claims	2,429,000
Homeowners [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 26,951	26,836	26,356	25,889	24,468	24,186		23,301	23,032
IBNR & development on reported claims	$ 63
Cumulative number of reported claims	2,694,000
Homeowners [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 37,978	37,880	37,185	35,859	35,834	42,021		43,807
IBNR & development on reported claims	$ 102
Cumulative number of reported claims	3,434,000
Homeowners [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 62,043	61,817	62,206	61,770	63,300	64,312
IBNR & development on reported claims	$ 144
Cumulative number of reported claims	7,657,000
Homeowners [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 94,703	94,760	95,129	92,411	99,497
IBNR & development on reported claims	$ 887
Cumulative number of reported claims	13,227,000
Homeowners [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 157,880	158,764	162,043	171,264
IBNR & development on reported claims	$ 4,709
Cumulative number of reported claims	24,219,000
Homeowners [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 188,548	192,769	202,844
IBNR & development on reported claims	$ 5,228
Cumulative number of reported claims	67,237,000
Homeowners [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 213,128	210,158
IBNR & development on reported claims	$ 9,975
Cumulative number of reported claims	36,555,000
Homeowners [Member] | Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 245,819
IBNR & development on reported claims	$ 58,908
Cumulative number of reported claims	17,670,000
Commercial General Liability [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 64,852
Commercial General Liability [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	8,401	8,252	7,809	7,645	7,597	7,535	$ 7,045	7,474	7,582	8,552
IBNR & development on reported claims	$ 106
Cumulative number of reported claims	761
Commercial General Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 7,817	6,828	6,254	5,409	4,760	4,603	4,749	5,854	6,436
IBNR & development on reported claims	$ 81
Cumulative number of reported claims	1,224
Commercial General Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 5,109	4,509	4,346	4,658	4,700	4,801	4,952	5,279
IBNR & development on reported claims	$ 94
Cumulative number of reported claims	712
Commercial General Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 5,984	5,580	5,704	5,502	5,221	5,069	7,095
IBNR & development on reported claims	$ 125
Cumulative number of reported claims	670
Commercial General Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 6,697	6,348	6,620	6,384	7,709	7,475
IBNR & development on reported claims	$ 149
Cumulative number of reported claims	761
Commercial General Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 7,947	5,377	6,020	7,008	8,082
IBNR & development on reported claims	$ 584
Cumulative number of reported claims	783
Commercial General Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 8,502	6,561	5,809	10,727
IBNR & development on reported claims	$ 858
Cumulative number of reported claims	743
Commercial General Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 6,558	7,853	8,289
IBNR & development on reported claims	$ 2,345
Cumulative number of reported claims	577
Commercial General Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 6,233	6,553
IBNR & development on reported claims	$ 4,395
Cumulative number of reported claims	388
Commercial General Liability [Member] | Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 1,604
IBNR & development on reported claims	$ 789
Cumulative number of reported claims	78
Personal Auto Liability [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 71,824
Personal Auto Liability [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	3,042	3,042	3,041	3,059	3,035	3,044	3,088	3,111	2,963	$ 2,823
IBNR & development on reported claims	$ 0
Cumulative number of reported claims	969
Personal Auto Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 2,762	2,756	2,796	2,848	2,762	2,912	2,954	3,350	$ 3,580
IBNR & development on reported claims	$ 0
Cumulative number of reported claims	789
Personal Auto Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 1,444	1,448	1,491	1,455	1,424	1,717	1,741	$ 1,735
IBNR & development on reported claims	$ 1
Cumulative number of reported claims	822
Personal Auto Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 2,127	2,123	2,161	1,829	1,826	1,863	$ 1,517
IBNR & development on reported claims	$ 5
Cumulative number of reported claims	3,471
Personal Auto Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 4,417	4,379	4,315	3,551	3,213	$ 2,038
IBNR & development on reported claims	$ 10
Cumulative number of reported claims	6,015
Personal Auto Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 2,915	2,878	2,781	2,882	$ 3,045
IBNR & development on reported claims	$ 8
Cumulative number of reported claims	6,538
Personal Auto Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 25,918	24,346	20,205	$ 13,414
IBNR & development on reported claims	$ 21
Cumulative number of reported claims	56,541
Personal Auto Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 24,579	22,472	$ 20,411
IBNR & development on reported claims	$ 243
Cumulative number of reported claims	42,064
Personal Auto Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ 4,623	$ 3,513
IBNR & development on reported claims	$ 600
Cumulative number of reported claims	7,975
Personal Auto Liability [Member] | Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred, claims and allocated claim adjustment expenses, net of reinsurance	$ (3)
IBNR & development on reported claims	$ 1
Cumulative number of reported claims	92